It is well known in the art, of electrographic, electrostatographic and electrophotographic copying and printing to form an electrostatic latent image corresponding either to the original to be copied, or to the digitized data describing an electronically available image, on a photoconductive member. In another image forming method, the electrostatic latent image is formed by imagewise discharge over styli towards a dielectric substratum. Another image forming method is called the xeroprinting process such as disclosed e.g. in European Patent Application 0 243 934 and it involves imagewise exposing a photopolymer master, charging e.g. by corona, toning with dry or liquid toner and transferring to another substrate. A survey of different methods for the production of electrostatic images on photoconductive electrically insulating recording materials and on non-photoconductive electrically insulating recording materials is given e.g. in U.S. Pat. No. 4,130,670.
Electrostatic latent images can be developed using a liquid developer consisting of a colloidal system of charged colloidal particles in an insulating liquid. In most cases the latent image is developed with a finely divided developing material or toner to form a powder image which is then transferred onto a support sheet such as paper. The support sheet bearing the toner powder image is subsequently passed through a fusing apparatus and is thereafter discharged out of the copying resp. printing machine as a final copy, resp. final print.
One of the objectives set forth for the overall electrostatographic process is to provide an image on the final copy, resp. final print with the best possible quality.
By `quality` in electrostatography is generally understood a true, faithful reproduction of the original to be copied, or faithful visual print of the electronically available image.
Quality consequently comprises features such as uniform darkness of the image areas, background quality, clear delineation of lines, as well as overall resolution of the image. The accuracy, inclusive of the resolution, with which the latent electrostatographic image, formed in either an electronic printing or copying apparatus, is developed into a visually discernable copy, is predominantly determined by the characteristics of the developer used.
It is known that one of the principal contributing characteristics herefore is the size and size distribution of the developer particles used, and in case a two component developer material is used, in particular the size and size distribution of the toner particles employed.
In the document published by ATR Corporation, 6256 Pleasant Valley Road, El Dorado, Calif. 95623, entitled `Effect of Toner Shape on Image Quality` published Mar. 28, 1988, the influence of toner particle diameter and shape upon image quality, particularly for high resolution images, has been tested.
For a developer to be used in a high resolution laser beam printer, the effects of toner particle size and shape upon the image were experimentally examined. As a result it was made clear that apart from the shape of the toner and its charge distribution, fine particles are most effective to provide high resolution.
This fact being known in itself, there have been several prior art proposals for the manufacture of fine toner particles and in particular for toner particles, the size distribution of which meets a well-defined classification.
In U.S. Pat. Nos. 3,942,979, 4,284,701, GB 2,180,948, EP-A 0 255 716 and in particular in EP-A-89201695.7 now publication number 479,875 such classified fine developers have been described.
One of the problems encountered with such fine developers, is the reduced efficiency of transferring the fine toner-image layer from the photoconductive surface to the image receiving substrate.
Such problem is e.g. explicitely recognised in EP-A-354 531 wherein is stated that the conventional electrophotographic process works well with large toner particles, but that difficulties arise as the size of the toner particles is reduced. Image defects such as "halo defect", "hollow character" and "dot explosion" arise. Thus, high resolution images require very small particles, but high resolution images without image defects have not been achievable using electrostatically assisted transfer.
In experiments it also has been noticed that when toner images present on the photoconductive drum are transferred in a conventional electrophotographic transfer station, the transfer efficiency decreases substantially when fine toner particles are used, as is required for obtaining high resolution images.
Whether the toner particles are prepared by conventional techniques, or by any alternative preparation method, as direct suspension polymerization techniques, spray drying, heterocoagulation, etc. the criticality in transfer behaviour described above does not alter.
In "Xerox Disclosure Journal" vol. 2, no. 5, September/October 1977, page 13 "Toner mixture for improved image quality" it is disclosed that a high transfer efficiency may be obtained when a bi-modal toner particle size distribution is employed. A preferred mixture of toner comprises particles of about 10-15 microns diameter and particles of about 4 microns or less. However, said disclosure contains no indication as to the respective amounts of smaller and larger toner particles which should be used for obtaining a high transfer efficiency. From experiments we have learned, as will be described hereinafter, that surprisingly, a high transfer efficiency and consequently a high image quality can only be obtained when particular ratios or amounts of smaller resp. larger particles are used.